1. Field of Invention
This invention relates to more efficient production of light from battery-powered flashlights by the addition of a voltage regulator circuit which supplies the incandescent lamp with relatively constant RMS voltage independent of battery voltage output. During start-up the regulator circuit in the present invention provides gradual power application to the incandescent lamp life reducing cold-filament thermal shock and thereby extending lamp life. Lamp life is further extended by the use of rectified AC power which reduces filament embrittlement. A voltage monitor circuit signals the approach of battery end-of-life by modulating the light output of the flashlight. A battery tester enables determination of the battery voltage which is indicated to the user by the flashing pattern of the light. An inherent power-off characteristic of the present invention eliminates the need for the commonly used high power on-off switch.
While this invention is intended primarily for the improvement of flashlight performance, it can also be used to advantage in other illumination products or other product applications where accurate delivery of root mean square, RMS, power to a resistive load is required.
2. Flashlights
Currently commercialized flashlights employ a direct mechanically switched connection between the battery power source and the incandescent lamp. This simple design approach provides a very low initial-cost solution which suffers from several shortcomings. All batteries of the type traditionally used in current flashlights exhibit relatively poor voltage maintenance characteristics over their useful life which results in substantial variability of light output and less that optimum energy conversion efficiency. Additionally, the current flashlight design approach is also subject to premature lamp and switch failures caused by very high cold-filament in-rush currents which exceed steady-state current levels by as much as a factor of ten.
Incandescent lamps produce visible light by resistive heating of the lamp filament to elevated temperatures in the range of 2200.degree. K to 3350.degree. K..sup.1 Most of the electrical power applied to the incandescent lamps is lost to thermal convection and emission in the infra-red spectrum with only a small fraction of the applied power producing the useful visible light. As the power applied to the lamp is increased, the filament temperature increases somewhat less than proportionately while the useful visible light increases very rapidly as described by the Stefan-Bolzman law.sup.2 of thermal radiation which states that "the power emitted from a black body thermal source is proportional to the fourth power of its absolute temperature."
Increasing the power applied to an incandescent lamp increases the filament temperature which accelerates the filament material evaporation rate. As the filament material evaporates with lamp usage, the filament strength deteriorates which ultimately leads to filament failure..sup.3 Additionally, the evaporated filament material becomes deposited on the relatively colder inside wall of the glass envelope blocking the transmission of the light produced by the filament. Long life incandescent lamps can also fail due to filament embrittlement rather than filament evaporation. As tungsten filaments are heated for long periods of time, the filament crystalline structure changes from a random long grain flexible structure to a more rigid short grain structure like glass. This change in the filament material structure to the more rigid short grain structure is commonly referred to as the filament notching effect. Even a slight vibration or shock can cause a notched filament structure to fracture and fail. It has been found that AC lamp operation decreases the filament notching effect two to ten times over that obtainable with pure DC operation. Rectified and unfiltered AC operation offers a compromise in filament life improvement between AC and DC operation as a result of the AC component which is superimposed on the average DC value..sup.4
All batteries which are commonly used in flashlights exhibit a loss of voltage output as the battery capacity is consumed with usage. The degree of the voltage loss varies with the type of battery chemistry and electrical load, however, the general shape of the battery voltage output function is relatively similar for all of the types of batteries typically used in flashlights. Fresh battery voltage output begins at a maximum, then drops quite quickly during the first part of the discharge cycle and then slows to a gradual decline for the bulk of the battery capacity. As the battery approaches its end-of-useful life, the voltage reduction rate increases continuously until the rate becomes precipitous at the end-of-life. Battery output voltage is also affected by the battery temperature, especially in the colder regions of operation. From room temperature downward the battery output voltage falls and at the same time the battery internal resistance increases compounding the reduction of the net available battery capacity. In addition, battery voltage output is also affected by the electrical load, and to a lesser extent the battery age, duty cycle, and recovery time.
As an example of typical voltage loss with usage, manganese-alkaline batteries under moderate load at room temperature provide approximately 1.25 volts per cell output when fresh and less than 0.7 volts per cell when the battery capacity nears exhaustion. An incandescent lamp of the type used in flashlights when powered directly by such a manganese-alkaline battery over the life of the battery exhibits a decrease of 7:1 in the light output, a decrease of 3:1 in the power to light conversion efficiency, and a decrease of 1:380 in the incandescent lamp wear out rate.
Room temperature incandescent lamp cold filament resistance is typically only 1/6th to 1/20th of that at the normal operating temperatures..sup.5 When power is first applied to a miniature incandescent lamp, a large current in-rush transient lasting typically only a few milli-seconds occurs which decays quickly as the lamp filament temperature rises to its normal operating value. The peak value of this in-rush current surge is usually somewhat less than would be indicated by the lamp cold-to-hot filament resistance ratio as the battery internal resistance and conductor inductance typically constrain the current delivery capability. In practice, flashlight in-rush currents have been measured to be in the range of 3-10.times. higher than the steady-state levels. Every time that a flashlight is turned-on, the in-rush current surge thermally shocks the lamp filament weakening the filament structure. The damage to the filament structure produced by the thermal shock is cumulative and can become the dominant lamp failure mechanism if frequent power-on cycling occurs. In addition to lamp life reduction, the large in-rush current transients also increase the current carrying demands placed on the power switch which can become quite costly if high product reliability is required.
The above described shortcomings with respect to current flashlight light generation efficiency and component reliability have been understood from the beginning, however it is only recently that the availability of low cost electronic components has made invention of power supply regulation circuits for improving flashlight performance commercially feasible.
3. Incandescent Lamp Performance.sup.6
The total light output of a miniature incandescent lamp in the visible region has been found through experimentation to be a function of the applied and design voltages as described by the equation: ##EQU1## where: L=Light output at V volts applied to lamp.
L.sub.d =Light output at design voltage V.sub.d PA1 V=Voltage applied to lamp. PA1 V.sub.d =Lamp nominal design voltage. PA1 K.sub.f =Constant between 3.2-3.7 determined experimentally for the particular lamp design. A typical value of 3.5 for K.sub.f will be used in throughout this disclosure. PA1 I.sub.d =Current at the design voltage V.sub.d. PA1 K.sub.c =Constant between 0.52-0.57, determined experimentally for the particular lamp design. A typical value of 0.55 for K.sub.c will be used throughout this disclosure. PA1 E.sub.avg =Average Efficiency
The corresponding relationship for lamp current has been found to be: ##EQU2## where: I=Current at V volts applied to lamp.
Multiplying both sides of Equation 2 by V/V.sub.d provides the expression for lamp power P with V.sub.r defined as the ratio of V/V.sub.d thereby normalizing the expression to the lamp nominal design voltage: EQU P.varies.V.sub.r !.sup.(1+K.sbsp.c.sup.) ( 3)
Dividing both sides of Equation 2 by V.sub.r according to Ohm's law provides the lamp filament resistance R: EQU R.varies.V.sub.r !.sup.(K.sbsp.c.sup.-1) ( 4)
Dividing light output Equation 1 by power consumption Equation 3 yields the light output efficiency E, proportional to Lumens/Watt: EQU E.varies.V.sub.r !.sup.(K.sbsp.f.sup.-K.sbsp.c.sup.-1) ( 5)
And lamp filament life had been found through experiments to obey the following proportionality: ##EQU3## where: J=An exponent constant in the range of 10-13 for the type of miniature incandescent lamps used in flashlights. A value of 12 for J will be used throughout this disclosure.
Substituting typical values for small incandescent lamp of 3.5 for K.sub.f and 0.55 for K.sub.c into the Efficiency Equation 5 results in: EQU E.varies.V.sub.r !.sup.1.95 ( 7)
Substituting typical value for small incandescent lamp of 12 into the Lamp Life Equation 6 results in: ##EQU4## The importance of incandescent lamp voltage regulation can be seen from Equations 7 and 8 which show that incandescent lamp light output efficiency increases at nearly the second power of the applied to design voltage ratio while life decreases at the 12th power of the same ratio.
4. Current Flashlight Performance
A representative flashlight design powered by the most commonly used manganese-alkaline batteries will be described to illustrate current flashlight performance. Subsequently, the current flashlight performance will be compared with a flashlight design using the present invention.
FIG. 1 shows the normalized voltage output curve of a manganese-alkaline battery of a conventionally designed flashlight when that battery is discharged by the direct connection to an incandescent lamp load. The 100% value on the vertical axis indicates the lamp nominal voltage operating point. Current flashlight design practice entails the use of a lamp with a nominal voltage rating which is somewhat lower that the fresh battery output voltage. Although this design practice increases the risk of premature lamp burn-out due to excess voltage when the batteries are fresh, the counterbalancing benefit realized is a substantial improvement in the average lamp output efficiency over the life of the battery as a result of the increased fraction of battery capacity that is discharged more closely to the incandescent lamp nominal voltage operating point.
The battery voltage discharge function shown in FIG. 1 has been derived from a battery manufacturer's.sup.7 "D" cell constant load discharge specification together with Equation 4 which is used to factor in the non-linear resistance characteristics of the incandescent lamp filament. Application of Equations 1, 2, 3, and 5 to the battery voltage output function yields the lamp light output, lamp current, lamp power, and light to power conversion efficiency functions respectively, which are also shown plotted in FIG. 1 with all values normalized to equal 100% at the lamp nominal voltage operating point.
The relatively rapid decline of the light output function with respect to the battery voltage function is the result of the 3.5 power exponent in Equation 1 which becomes applied to the instantaneous battery output voltage. The Lumens/Watt power conversion efficiency function declines somewhat more slowly than the light output function as the lamp power consumption also declines somewhat with the declining battery output voltage. The average of the battery-lamp power conversion efficiency over the life of the battery is indicated by the horizontal line. The difference between 100% power conversion efficiency of an ideal design and the average power to light conversion efficiency of the conventionally designed manganese-alkaline battery powered flashlight defines the theoretical maximum light production improvement opportunity that might achieved with improved voltage control: ##EQU5## where: I.sub.opp =Maximum Improvement Opportunity
For the manganese-alkaline flashlight design example presented in FIG. 1, the average power to light conversion efficiency was shown to be 63%. Substituting that average power to light conversion efficiency into Equation 9 shows that theoretical maximum opportunity for power to light conversion efficiency improvement with improved voltage control is therefore 58%.
5. Prior Art
A voltage regulating circuit to permit constant light output level to be efficiently produced from an incandescent lamp over a wide range of source voltage is disclosed in U.S. Pat. No. 4,230,970. This regulating circuit is described as producing voltage regulation within +/-0.5% over an input voltage range of 6 to 12 Volts by the controlled periodic application of the full source voltage to the incandescent lamp. This regulating circuit utilizes an analog circuit method for providing a mathematical function which controls the timing of a pulse width modulated constant frequency voltage pulse train which is applied to the incandescent lamp. The on-fraction of the pulse width timing is established by the time required for the charging of a timing capacitor to a pre-set constant voltage level by the varying voltage source. When the voltage on the timing capacitor reaches the constant pre-set voltage level, the voltage applied to the lamp is turned-off and the timing capacitor is then discharged. The off-fraction of the pulse train applied to the lamp is determined by the difference between a free-running oscillator time period and the time used for the charging of the timing capacitor.
Another regulating circuit to permit relatively constant light level to be efficiently produced by an incandescent lamp over a wide range of source voltage is disclosed by U.S. Pat. No. 4,237,405. This regulating circuit shares many similarities with the regulating circuit in U.S. Pat. No. 4,230,970 above, varying primarily in the design of the circuit used for the charging and discharging of the timing capacitor and by the replacement of the free-running oscillator with a relaxation oscillator. This circuit is also described as producing regulation within .+-.0.5% over an input voltage range of 6 to 12 Volts.
Another regulating circuit to permit constant light level to be efficiently produced from an incandescent over a wide range of source voltage is disclosed by U.S. Pat. No. 4,499,525. This regulating circuit has several similarities to the two U.S. Pat. Nos. 4,230,970 and 4,237,405 discussed above including the use of pulse-width application of the full source voltage to the lamp to produce constant RMS power and the use of a timing capacitor for the establishment of the pulse train duty cycle, but differs in the design of the timing capacitor charging and discharging circuit and the oscillator circuits. This regulating circuit utilizes three forward biased diode junctions in series for the establishment of the constant pre-set voltage level used for setting the triggering level of the timing capacitor. The triggering function is implemented with two transistors which, in combination with the forward drop across the three diodes, establish the capacitor voltage triggering level that in turn determines the RMS conversion accuracy. This method and apparatus is shown through measurements to produce regulation within .+-.7% over an input voltage range of 3 to 6 Volts.